1. Field of the Invention
The present invention relates to a visible-light communication technique that achieves bidirectional visible-light communication.
2. Description of the Related Art
In recent years, the development of visible-light communication technology has progressed, which utilizes the visible light emitted from a light source such as a light emitting diode (hereinafter occasionally referred to as an “LED”). The visible light coming from the light source reaches a receiving side, while it is diverging. It is inevitably difficult for the receiving side to detect the position of the light source with high accuracy and to return the visible light to the transmitting side, while focusing the light at the transmitting side. This is why unidirectional communication is performed in the visible light communication, from the transmitting side to the receiving side. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-202741).
In order to accomplish bidirectional communication using visible light, it is proposed that a hybrid system be utilized, in which the visible light communication is combined with infrared-ray communication wherein the light hardly diverges. With the hybrid system, however, the infrared-ray source must be manually or automatically controlled and the visible-light emitting side must have a focusing mechanism for focusing the infrared rays before emitting them. If provided with an infrared-ray source and such a focusing mechanism, the hybrid system will be a bidirectional communication system of complex configuration and high manufacturing cost.
A single-light-source system having two communication apparatuses is proposed, in which one apparatus has a light source that emits visible light and the other apparatus reflects the visible light, thereby transmitting data to the first-mentioned apparatus. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-221747). The single-light-source system needs to have neither an infrared-ray source nor a focusing mechanism. It can therefore provide a bidirectional communication system that has a simple configuration and can be manufactured at low cost. Moreover, the other apparatus need not have a power supply for a light source and can be used as a mobile terminal of low power consumption.
The single-light-source system can indeed achieve bidirectional, visible light communication using communication apparatus, such as a mobile terminal of low power consumption. If data is transmitted by the use of reflected light, however, a communication trouble, such as interference, will occur because the visible light emitted from the transmitting side has data superimposed on it in the single-light-source system.